Trolleybuses would only work on the kind of congested urban routes that tend to see the newest double deckers.
Either you'd end up keeping these buses on these routes for the whole of their life (which would mean Oxford Road having knackered old buses with poor facilities in a generation's time) or you'd cascade them elsewhere.
Cascading them elsewhere would see other parts of Manchester/ UK lose their modern buses for these mid-life Oxford Road buses - it's one thing to have 308s replacing Pacers on the railways, but I can't see that working on bus routes.
The outcry against the Cambridgeshire Guided Busway was bad enough without it being run with fifteen year old buses (that are much older than the ones they replaced).
Well since conventional bus orders would likely slow drastically in a reasonable world that went for big time trolleybus orders, the numbers of "modern buses" that would be around in 10-15 years time when the Oxford road buses need to be disposed of would be rather small.
And the Oxford Road corridor has M and W-reg buses on it now on the Magic Bus services.
And 78 double decker buses per day each direction is a sufficiently low figure that I imagine there are several routes not in the most stereotypical urbanised locations that come close to qualifying once you consider network effects.
Also one of the major problems wiht older buses is smells and vibrations from the engine bay due to failing seals and the like, this is not a problem with a trolleybus, similar in principle to how diesel trains have shorter useful lives than electric ones.
EDIT:
Also there is the question of whether you would need double lane electrification on the entire bus network, it would be interesting to see whether lightly used routes that share significant amounts of road with heavily used trolleybus lines could be supplied with short sections of "single" trolleybus equipment with baloon loops on the end to permit the buses to turn around.
That cuts effective costs at-least in half because there would be no need for switchwork for significant distances.